I guess that makes sense, thanks for the clarification!
§ 15.23 Home-built devices.
(a) Equipment authorization is not required for devices that are not marketed, are not constructed from a kit, and are built in quantities of five or less for personal use.
(b) It is recognized that the individual builder of home-built equipment may not possess the means to perform the measurements for determining compliance with the regulations. In this case, the builder is expected to employ good engineering practices to meet the specified technical standards to the greatest extent practicable. The provisions of § 15.5 apply to this equipment.
https://www.ecfr.gov/current/title-47/chapter-I/subchapter-A...
This entire part from the FCC basically states you don't need a license to operate in the frequencies for wifi, bluetooth, etc. You're not breaking a law by recompiling the firmware for your wifi module to fix a bug. You'd be breaking the law if you did so with the intention of operating within licensed spectrum/power levels, for example.
So, let's say you modify something with your own firmware, break rules about ISM spectrum - or worse, mess with SDR hard enough you break some licensed spectrum, and upon investigation FCC certification marks are found and the number. Since certification points to vendor, vendor now has to explain why their device went outside of those limits, and might or might not be able to prove that you ran it with unlicensed firmware.
So an obviously home build device will go under §15.23 easily, but inconspicuously modified commercially sold device won't - without possibly long court case, that is.
And if this really was really the main issue, it seems pretty easy to just sign the firmware - I'm pretty sure many vendors do it already.
With significant portion of the regulated behaviour being done in software, things can become a bit problematic if the end user can load any code they want. This is also why "BIOS whitelists" exist, as the certification applies to the whole radio equipment, which means the certification must cover the antenna - and those are built into laptops, meaning you can't certify the cards separately as their exact characteristics depend on the connected antennas.
